After the violent death of its messiah, Christianity must have seemed destined for obscurity. But instead of disappearing, it grew — from a few thousand souls to about 30 million in just 300 years. How in heaven’s name did it happen?

That’s the big question behind biblical scholar Bart D. Ehrman’s erudite and engaging new book. As an academic historian, he won’t accept appeals to divine providence. At the same time, he’s also skeptical about common secular answers, such as the idea that the Roman Empire became Christian as a direct result of the emperor Constantine’s conversion in A.D. 312. “Until recently, that is what I myself thought. But I no longer think so,” Ehrman admits. “On the contrary: I think Christianity may well have succeeded even if Constantine had not converted.”

Why? Ehrman argues that Christianity differed from older religions in two important ways. Unlike Judaism, it was missionary. And unlike pagan religions, it was exclusive. If you were a Christian, you could worship no other gods.

The Roman world was full of divine beings. There were “gods connected with love, war, livestock, crops, health, childbirth, and weather,” writes Ehrman. “Gods of various abstractions, such as fortune, mercy, and hope; gods connected with elements of nature, like the moon, the sun, the sky, and the sea.” Each of these gods had its own cults and rituals, but people could participate in as many as they liked.

Christians, on the other hand, promised to worship only one God. So as Christianity waxed, the pagan population waned. “Unlike any religion known to the human race at the time,” Ehrman writes, Christianity elbowed aside other faiths. As the old religions lost their popular and financial support, they withered away.

In this contest for religious market share, the pagans didn’t advertise; the Christians did. Early Christians “used their everyday social networks and converted people simply by word of mouth.” They told stories about Jesus and his disciples and their miraculous deeds. In fact, they themselves had experienced this wonderworking power in their own lives — or had at least heard about someone who had. Most of their friends would probably nod politely, but a few might find the stories intriguing. If even one converts, “that opens up more possibilities of sharing the ‘good news,’ because she too has friends. And family, neighbors, and people she sees in all sorts of contexts.”

Could word of mouth really spread Christianity across the Roman Empire? Ehrman believes that the answer is yes. Based on statistical modeling, he shows that even a modest growth rate — say 2.5 percent a year — would eventually lead to exponential numerical growth, like a savings account accruing compound interest. “I need to stress that we are not talking about implausible rates of growth,” he insists. “Every hundred Christians would need to convert just two or occasionally three people a year.” Important as Constantine was, by the early fourth century he was just one part of a larger demographic wave.

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Ehrman’s conclusions are debatable, as he knows perfectly well. Like a good college lecture class, his book offers both a wealth of historical information and, to make sense of it all, a few plausible theories — including his own. He doesn’t tell us what to think. He gives us a lot to think about.